In recent years, worldwide concern has arisen relating to climatic changes ascribed to the increase in atmospheric carbon dioxide released by fossil fuels. The price of such fuels has been increasing, and projections as to the remaining world supplies of such fossil fuels have led to increased interest in the development of devices utilising alternative fuels. Furthermore, use of renewable fuels in such devices could slow down the increase in carbon dioxide levels in the atmosphere.
Devices, which burn wood are known. However, cord wood burns inefficiently in conventional combustion chambers and is inconvenient to use in comparison with oil or gas burning devices.
The use of biomass fuels is also increasing. However, again, biomass fuel tends to burn inefficiently in conventional devices and this is due to its high volatiles content. Much of the heat is released into the burning gases (the flames) and is lost up the chimney or flue.
Another reason that conventional boilers and stoves are unsuitable for burning pelleted biomass fuels such as wood pellets, wood chips, etc., is that such pellets tend to smoulder at reduced output particularly when they have a relatively high moisture content. Consequently devices have been developed for burning these fuels more efficiently.
Thus, for example, a typical wood pellet stove includes a hopper, an auger, a firebox or grate, a combustion fan and a heat exchanger which, respectively, store, feed, burn the fuel and transfer the heat to the space to be heated. The auger operates in a timed manner to control the delivery of the pellet fuel from the hopper into the firebox. The rate of delivery of the fuel to the firebox is matched to the rate of consumption of the fuel for a particular output. The combustion fan provides a measured amount of air to the firebox. An example of such a stove is the Pellet stove Mod. 1000 manufactured by Caminetti Montegrappa s.r.l of via A. da Bassano, 7/9, 36020 Pove Del Grappa (VI), Italy.
However, a problem with such stoves and boilers is that the transfer of heat from the burning volatiles is relatively low, particularly at low output, and acceptably high efficiencies can only be achieved by passing the flue gases through extensive heat exchangers.
German Patent Publication No. DE 92 18 953 describes a pellet burning stove in which the pellets are burned in an open burner pan located at the base of a combustion chamber. Complete combustion of the pellets is achieved in the combustion chamber, and the hot exhaust gases exit through holes in the top of the combustion chamber and are directed through a heat exchanger to extract the heat therefrom.
U.S. Patent Application Publication No. 2007/0089733 describes a wood-burning boiler having a combustion chamber with an air intake and an exhaust chamber connected to the combustion chamber for receiving burnt gases through an exhaust opening therein. The boiler includes first and second air preheat chambers from which secondary air is introduced into the combustion chamber through a plurality of holes. The exhaust chamber is positioned next to the second air preheat chamber such that in use heat from the exhaust gases is transferred to the air in the preheat chamber before the exhaust gases pass from the exhaust chamber into an exhaust passage where further heat is extracted by means of a water jacket surrounding the exhaust passage.
It is an object of the present invention to overcome the disadvantages of the devices hereinbefore described.